Spectral resolution of fluxional organometallics. The observation and FTIR characterization of all-terminal [Rh4(CO)12]

Abstract

In situ FTIR spectroscopy at 1 cm⁻¹ resolution was conducted on n-hexane solutions of the bridged [Rh₄(CO)₉(μ-CO)₃] in the interval T = 268–288 K and P_T = 0.1–7.0 MPa using either helium or carbon monoxide as dissolved gas. Analysis of the spectral data sets was conducted using band-target entropy minimization (BTEM), in order to recover the pure component spectra. A new spectral pattern was recovered with terminal vibrations at 2075, 2069.8, 2044.6 and 2042 cm⁻¹. The new spectrum is consistent with an all-terminal [Rh₄(CO)₁₂] species with a C_3v anticubeoctahedron structure where 2 different [Rh(CO)₃] moieties exist, although the presence of some T_d structure can not be entirely excluded. The equilibrium between all-terminal [Rh₄(CO)₁₂] and the bridged [Rh₄(CO)₉(μ-CO)₃] was determined in the presence of both helium and CO. The equilibrium constant K_eq = [Rh₄(CO)₁₂]/[Rh₄(CO)₉(μ-CO)₃] at 275 K was ca. 0.011 and the determined equilibrium parameters were Δ_rG = 12.63 ± 4.8 kJ mol⁻¹, Δ_rH = −21.45 ± 2.3 kJ mol⁻¹ and Δ_rS = −114.3 ± 8.35 J mol⁻¹ K⁻¹. The free energy indicates a very small difference between the bridged and terminal geometry, and the lower entropy is consistent with a higher symmetry. This finding helps to address a long-standing issue concerning the existence of various [M₄(CO)₁₂] symmetries. In a more general context, the present study illustrates the considerable utility of quantitative infrared spectroscopy (occurring on a fast vibrational timescale) combined with sophisticated deconvolution techniques in order to resolve systems which have been demonstrated to be fluxional on the NMR timescale.